Often blows from the window, which closed absolutely tight and no cracks. It seems strange. Meanwhile here is not surprising.
The air in the room is almost never is alone; there are invisible currents caused by heating and cooling. From heating the air razreshaetsja and, therefore, becomes easier; on cooling, in contrast, is compacted, it becomes harder. Easy hot air from radiators or warm oven replaced by cold air up to the ceiling, and the air cooled, heavy, near Windows or cold walls, dripping down to the floor.
These currents in the room easy to find with child a balloon, if you tie to it a small load to the ball rested at the ceiling, and freely floating in the air. Released near the heated stove, so the ball travels around the room, dragged the invisible air currents from the stove under the ceiling to the window, there falls to the floor and returned to the stove again to travel around the room.
That is why in winter we feel the blows from the window, especially in the legs, although the frame so tightly closed that outside air cannot pass through the slit.